The invention aims at providing an improved method and device, in such a way that the gain setting of the amplifier is more in conformity with the background noise in the space thereby obtaining a more pleasant level of the desired acoustic signal in the space.
To this end a method in accordance with the invention is characterized in that, for a specific gain setting of the amplifier and specific contents of at least one memory corresponding to an associated frequency range, the following set of steps is carried out in each of successive operations:
(a) a sample comprising a portion of the electric signal which lies within a specific time interval is taken, PA1 (b) a quantity is derived from said sample of the electric signal for said frequency range, which quantity is a measure of the energy content of said portion in the frequency range, PA1 (c) the quantity corresponding to the energy content in the freqeuncy range is compared with the value of the contents of the memory, PA1 (d) if it is found in step (c) that the value obtained for the quantity in step (b) lies within a range of values around the value stored in the memory, the count of a counter which corresponds to the frequency range is incremented by a first number and subsequently the count of this counter is compared with a first final value, PA1 (e) if it is found in step (c) that the value obtained for the quantity in step (b) lies outside the range of values around the values stored in the memory or if it is found in step (d) that the count of the counter has reached the first final value, the count of the counter is decremented, preferably set to a first initial value, the operation is repeated from step (a) for a new sample comprising a portion of the electric signal which lies within a specific time interval and, if during an operation it is found for the frequency range that the count of the counter has reached the first final value, the gain of the amplifier is adapted (if necessary). If the counter is set to the first initial value in step (e), it is clear that the said counter indicates the length of the sucession of immediately preceding operations for which the quantity lay within the range around the value stored in the memory. PA1 (a) a gain which is far too high in comparison with the "true" background noise, i.e. the acoustic signal in the space minus the desired signal delivered by the amplifier and minus the speech, PA1 (b) a gain which is much too high in comparison with the speech level. PA1 (i) viewed in time, speech signals fluctuate rapidly in amplitude, whilst the "real" background noise is of fairly constant amplitude; PA1 (ii) viewed in time, the frequency of speech changes rapidly and the frequency (band) of the "real" background noise changes hardly or not at all (or in any case very slowly); PA1 (iii) speech signals exhibit pauses (to build up explosions during the pronunciation of some sounds, for example the letter "p"), whilst the "real" background noise generally exhibits no breaks. PA1 (a) a sample comprising a portion of the electric signal which lies within a specific time interval is taken, PA1 (b) a quantity is derived from this sample of the electric signal for a frequency range i, which quantity is a measure of the energy content of said portion in the relevant frequency range, PA1 (c) the i.sup.th quantity corresponding to the energy content in the i.sup.th frequency range is compared with the value of the contents of the i.sup.th memory of the n memories each of which corresponds, to a respective frequency range, PA1 (d) if it is found in step (c) that the value for the i.sup.th quantity obtained in step (b) lies within a range of values around the value stored in the i.sup.th memory, the count of one of n counters each of which corresponds to an associated frequency range, is incremented by a first number, and, subsequently, the count of this counter is compared with a first final value, PA1 (e) if in step (c) the value for the i.sup.th quantity obtained in step (b) is found to lie outside the range of values around the value stored in the i.sup.th memory or if in step (d) the count of the counter is found to have reached the first final value, the count of the counter corresponding to the i.sup.th frequency range is decremented and preferably set to a first initial value, PA1 (f) if it is found in step (d) that the count of the counter has not yet reached the first final value or if step (e) is carried out, the value i is incremented by a second number and compared with a second final value, PA1 (g) if it is found in step (f) that the value i has not yet reached the second final value, the method is continued from at least step (c) for another frequency range, PA1 (h) if it is found in step (f) that the value i has reached the second final value, the operation is repeated from step (a) for a new sample comprising a portion of the electric signal which lies within a specific time interval, the value for i being set to a second initial value before step (c) is carried out again, and if during an operation the count of the associated counter is found to have reached the first final value for at least one frequency range i, the gain of the amplifier is adapted (if necessary). In this case it is clear that, if the counters are set to the first initial value(s) in step (e), the said counters each indicates the number of immediately preceding operations in which for the corresponding frequency range i, the value obtained for the quantity lay within the range around the value stored in the i.sup.th memory. This method in addition utilizes the aspect given in point (ii). PA1 an amplifier whose gain can be controlled depending on a control signal and having an input coupled to a first input terminal for receiving an electric signal to be amplified, an output coupled to an output terminal for supplying an output signal, and a control input for receiving the control signal, PA1 a second input terminal for receiving an electric signal which is a measure of the acoustic signal in the space, PA1 conversion means for deriving a control signal which is a measure of the background-noise level, having an output coupled to the control input of the amplifier, PA1 correction means for at least substantially correcting for the influence of the signal which has been amplified by the amplifier on the control signal, is characterized in that the device further comprises PA1 a first unit having an input coupled to the second input terminal, for deriving for a frequency range from a portion of the electric signal which lies within a specific time interval a quantity which is a measure of the energy content of said portion in the relevant frequency range, PA1 a second unit having an input coupled to an output of the first unit and an output, which second unit comprises a memory corresponding to the frequency range, for storing a value which is a measure of the energy content of the electric signal in the frequency range, PA1 a first comparator having a first input and a second input coupled to the output of the first unit and the output of the second unit, respectively, for comparing the value for the quantity corresponding to the frequency range and derived in the first unit with the value of the contents of the memory of the second unit which corresponds to said frequency range, PA1 a third unit comprising a counter corresponding to the frequency range, PA1 a second comparator having a first input coupled to an output of the third unit and an input for the first final value, which second comparator is adapted to compare the count of the counter with the first final value, the first comparator has a first output and a second output for supplying an output signal if the value of the quantity determined in the first unit lies within and outside, respectively, a range of values around the value stored in the memory of the second unit, said first output and the second output are coupled to a count input and another input preferably a reset input, respectively, of the third unit, the output of the second comparator is coupled to the other input of the third unit and is also coupled to a control input of the conversion means, for supplying a control signal if it is found in the second comparator that the count of the counter in the third unit has reached the first final value. PA1 a fourth unit comprising n memories each corresponding to a respective one of the n frequency ranges, for storing n reference values each corresponding to the respective one of the n frequency ranges, PA1 a fifth unit which also comprises n memories each corresponding to a respective one of the n frequency ranges, PA1 a sixth unit having a first input and a second input coupled to an output of the fourth unit and to an output of the fifth unit, respectively, and having an output coupled to the output of the conversion means, which sixth unit is adapted to derive a control signal from the signals applied to its two inputs, the control input of the conversion means is coupled to a load input of the fifth unit, for storing the if applicable, new value for the i.sup.th quantity in the corresponding memory of the fifth unit if it is found in the second comparator that the count of the i.sup.th counter in the third unit has reached the first final value. The construction of the sixth unit depends on whether the amplifier has a gain which is variable and adjustable over the entire audio-frequency range or a gain factor which is adjustable and variable in a plurality of m (which may be equal to n) frequency ranges. PA1 a second unit having an input and an output, which second unit comprises a memory which corresponds to the frequency range for storing a value which is a measure of the energy content of the electric signal in the frequency range, PA1 a first comparator having a first input and a second input and an output, which second input is coupled to the output of the second unit, for comparing the value for the quantity corresponding to the frequency range with the value of the contents of the memory of the second unit corresponding to this frequency range, PA1 a third unit comprising a counter corresponding to the frequency range, PA1 a second comparator having a first input coupled to an output of the third unit and an input for the first final value, which second comparator is adapted to compare the count of the counter with the first final value, the first comparator comprises a first output and a second output for supplying an output signal if the value of the quantity lies within and outside, respectively, a range of values around the value stored in the memory of the second unit which first and second outputs are coupled to a count input and to another input preferably a reset input, respectively, of the third unit, the output of the second comparator is coupled to the other input of the third unit and is also coupled to a control input of the conversion means, for supplying a control signal if it is found in the second comparator that the count of the counter in the third unit has reached the first final value.
The invention is based on the recognition of the fact that the methods and devices known until now are not capable of distinguishing between background noise and speech.
To illustrate for know methods and devices, if two persons in the space (for example, the passenger compartment of a car) in which a radio employing said known method or equipped with said known device is present and is switched on, begin a conversation, the method or the device will interpret the speech as background noise, which causes the gain of the amplifier to be increased.
Consequently, the persons will start talking louder, which results in a further increase of the amplifier gain. The persons again begin to talk louder, so that the gain is increased again . . . etc. This results in.
This is very annoying. Therefore, a system is required which responds to the "real" background noise and which does not respond to an undue extent to speech signals. This can be achieved by means of the inventive method. Use is made of knowledge that:
These three points impose special requirements on the method and the device. The step in accordance with the invention in which repeatedly measurements are made in (at least) one frequency range and in which it is detected whether the energy content in the frequency range varies is based on the aforementioned point (i).
It is evident that, if measurements in only one frequency range are made, this frequency range should contain the principal frequency components of the background noise.
An improved method in accordance with the invention may be characterized in that, for a specific setting of the gain of the amplifier and specific contents of n memories each of which corresponds to a respective one of n frequency ranges (n.gtoreq.2), the following set of steps is carried out in each of successive operations:
Hereinafter, a version for two or more frequency ranges will be described in more detail. However, it is obvious that steps relating to one frequency range may also be employed in a method in which measurements in only one frequency range are made. The frequency ranges should be selected in such a way that, for an optimum control in response to the "real" background noise level, they together cover a frequency range within which the "real" background noise lies at least for the greater part. Moreover, each frequency range should have only a part of the frequency range within which the speech lies in common with this frequency range.
If only speech signals are received in the frequency ranges, the signal content in the frequency ranges will vary rapidly as a function of time. However, if only the "real" background noise is detected, the signal content in one or more frequency ranges will remain substantially constant as a function of time. Thus, if in a plurality of successive operations in a frequency range i (and of course in parallel for the other frequency ranges) the i.sup.th quantity) generally referred to as "signal content" in order to indicate that at option, for example the energy content, the average amplitude or the RMS value may be determined) is determined it is possible to make a distinction between "real" background noise and "real background noise with superimposed speech". In the former case the gain of the amplifier may be adapted. In the latter case the gain should not be adapted.
This "distinction" is made as follows: if in a number of successive operations, which number corresponds to the first final value, it is found that the value for the i.sup.th quantity always lies within the range of values around the value stored in the i.sup.th memory (i.e. there is "real" background noise), the gain of the amplifier is adapted (if necessary). The level of the "real" background noise may not have changed since the last adaptation of the gain. Then, it is not necessary to adapt the gain. Of course, the gain may be nominally adapted. However, the gain is then maintained at the same value.
The control signal may not be only used for adapting the gain but also for adapting, for example, the dynamic range of the signal to be amplified. For example, the dynamic range may be reduced or extended when the background-noise level increases and decreases, respectively.
In those cases in which, before the first final value is reached, the value for the i.sup.th quantity lies outside the range of values around the value stored in the i.sup.th memory, the count of the counter corresponding to the i.sup.th frequency range is decremented and is preferably set to the first initial value. Consequently, the amplifier gain is not adapted. In this case there is apparently "real background noise with superimposed speech".
The gain may be adapted every time in step (d) immediately after it has been found that for a specific frequency range in this step i the count of the i.sup.th counter has reached the first final value. It is alternatively possible not to adapt the gain until the end of an operation for all those frequency ranges (at least one-) for which during the operation the count of the associated counter(s) is (are) found to have reached the first final value.
The first final value has a lower limit which is determined by the maximum duration of a speech utterance and the measurement time of an operation. The maximum duration of a speech utterance is approximately 250 ms and the measurement time of an operation depends inter alia on the delay times in the components used. The first final value should then be larger than or equal to 250 ms divided by the measurement time of an operation. The measurement accuracy increases as the first final value increases, but this leads to an increased response time of the system.
If the gain of the amplifier has the same value for the entire frequency range of the audio signals, the gain for the entire frequency range will be adapted even if the first final value is reached in only a single frequency range i. This means that acoustic and/or system-engineering steps are applied to enable the gain for the entire frequency range to be adapted although the first final value is reached only for a limited number of frequency ranges.
In accordance with the invention the method of adapting the frequency-dependent gain of the amplifier in said n frequency ranges is characterized in that if during an operation it is found for at least one frequency range that the count of the associated counter has reached the first final value, the gain of the amplifier in the i.sup.th frequency range is adapted (if necessary). This "one to one" relationship between the quantities corresponding to the energy content in the n frequency ranges and the gain of the amplifier in these n frequency ranges therefore enables the gain in the associated frequency range to be adapted (if necessary) every time that the first final value is reached for a frequency range.
However, other variants are also possible, for example those in which the gain of the amplifier can be adapted in m frequency ranges (m&lt;n). Now there is a relationship between the n frequency ranges in which measurements are made (and which have been combined to form m groups of frequency ranges, each group being related to one of the aforementioned m frequency ranges) and the m frequency range in which the gain is controlled.
A first embodiment of the method may be characterized in that, if it is found in step (c) that the value obtained for the i.sup.th quantity in ste (b) lies outside the range of values around the value stored in the i.sup.th memory, a new value for the i.sup.th quantity is derived in step (c) and is stored in the i.sup.th memory, starting from the value for the i.sup.th quantity obtained in step (b) and, as the case may be, the value of the contents of the i.sup.th memory. As the value for the i.sup.th quantity lies outside the range of values around the value stored in the i.sup.th memory, the conclusion must be drawn that there is either a varying background level or that the measurement of the background level has been disturbed by the presence of speech signals. These two possibilities cannot readily be distinguished from one another. Nevertheless, in the case of the first-mentioned possibility the value stored in the i.sup.th memory should be adapted to the new level measured. This may be achieved by storing the value for the i.sup.th quantity obtained in step (b) in the i.sup.th memory. In order to reduce both the influence of the presence of speech signals (the last-mentioned possibility) and the influence of extreme variations in background level, it is preferred to derive a new value for the i.sup.th quantity from the value of the i.sup.th quantity obtained in step (b) and the value of the contents of the i.sup.th memory (for example by taking the average of the two) and to store this value as the new vaue in the i.sup.th memory. In principle, the value stored in the i.sup.th memory need not be changed if the value of the i.sup.th quantity lies within the range of values around the value stored in the i.sup.th memory.
A second embodiment of the method may be characterized in that the step (c), starting from the value for the i.sup.th quantity obtained in step (b) and/or the value of the contents of the i.sup.th memory, also a new value for the i.sup.th quantity is derived and is stored in the i.sup.th memory. In this case, also if the value of the i.sup.th quantity lies within the range of values around the value stored in the i.sup.th memory, a new value for the i.sup.th quantity may be derived from the original value for the i.sup.th quantity and the value stored in the i.sup.th memory, which new value is stored in the i.sup.th memory. In this way it can be achieved that an as accurate as possible average background level is obtained.
A method in accordance with the invention may be characterized further in that in step (b) for all frequency ranges the associated quantities are derived and, if in step (f) it is found that the value i has not reached the second final value, the method proceeds from step (c). In one operation in accordance with this method a signal sample is taken only once and the corresponding quantities for all the n frequency ranges are determined at one time. This may be achieved, for example, in that a filter bank comprising n band-pass filters corresponding to the n frequency ranges is used and the output signals of these filters are measured at one time. Another possibility is to apply a Fourier transform (for example, a fast Fourier transform) to the signal sample taken in step (a) and to derive the quantities for the n frequency ranges from the result thereof.
Instead of this, the inventive method may be further characterized in that in step (b) said quantity is derived for only one frequency range i and, if it is found in step (f) that the value i has not yet reached the second final value, the method is continued from at least step (b) by determining the quantity which is a measure of the energy content of the relevant signal portion in another frequency range, and in step (h), if the operation is repeated from step (a), prior to step (b) being repeated, the value for i is set to a second initial value. In this case there are two possibilities of carrying out the method. In the case of the first possibility step (a) is carried out only once in one operation, after which n cycles are performed in each of which the i.sup.th quantity is derived for only one frequency range in step (b). This possibility may be adopted if a memory is used in which the signal sample can be stored. The reason for thus carrying out the method may be that only one filter is available whose central frequency and bandwidth are adjustable to the central frequencies and bandwidth of the n frequency ranges. In the case of the second possibility n cycles are performed in one operation, step (a) being carried out and subsequently in step (b) the i.sup.th quantity being derived from the signal sample for a frequency range i obtained in step (a) in one cycle. In a following cycle step (a) is repeated and the quantity for the (i+1).sup.th frequency range is derived. This possibility should be adopted if there is no memory for the storage of the signal sample.
The device for carrying out the method in accordance with the invention, comprising
If the device is for carrying out the method in which measurements are made in two or more frequency ranges, it may be characterized further in that the first unit is adapted to derive for a frequency range i, from the portion of the electric signal which lies within the time interval, the quantity which is a measure of the energy content of said portion in the relevant frequency range, the second unit comprises n memories each corresponding to a respective one of the n frequency ranges, each for storing a value which is a measure of the energy content of the electric signal in the associated frequency range, the first comparator is adapted to compare the value for the quantity corresponding to the i.sup.th frequency range and derived in the first unit with the value of the contents of the memory of the second unit corresponding to this frequency range, the third unit comprises n counters each corresponding to a respective one of the n frequency ranges, the second comparator is adapted to compare the count of the i.sup.th counter with the first final value, the first comparator is further adapted to supply respective output signals if the value of the quantity determined in the first unit and corresponding to the i.sup.th frequency range lies within or outside a range of values around the value stored in the i.sup.th memory of the second unit respectively, and the second comparator is further adapted to supply a control signal if it is found in the second comparator that the count of the i.sup.th counter in the third unit has reached the first final value.
As stated in the foregoing, if the value for the i.sup.th quantity lies outside the range of values around the value stored in the i.sup.th memory of the second unit, the content of the i.sup.th memory should be replaced by a new value derived from the value for the i.sup.th quantity and, as the case may be, the value of the contents of the i.sup.th memory. To this end the device may be further characterized in that the second output of the first comparator is coupled to a load input of the second unit for storing said new value, which appears on the input of the second unit.
In general, the values stored in the memories of the second unit are preferably adapted every time. Thus, this is also done if the value for the i.sup.th quantity lies within the range of values around the value stored in the i.sup.th memory of the second unit. The device in accordance with the invention may then be characterized in that the output of the first unit and the output of the second unit are coupled to a first input and a second input, respectively, of a signal combination unit, which has an output coupled to the input of the second unit, which signal combination unit is adapted to derive a signal from the signals on its two inputs and to produce the resulting signal on its output, and the second unit comprises a load input for receiving a control signal for the storage of the signal appearing on the input of the second unit in the associated memory i of the second unit.
The device may be characterized further in that it further comprises an address counter having an output coupled to an address input of the first to the third unit inclusive, and a clock pulse input which is coupled to an output of a clock-pulse generator.
The device may be characterized further in that the first unit for determining the quantity which is a measure of the energy content in a specific frequency range i comprises a bandpass filter having a bandwidth which at least substantially corresponds to the bandwidth of the frequency range, having an input coupled to the input of the first unit and an output coupled to an averager. For each of the n frequency ranges the first unit may comprise such a combination of a band-pass filter and an averager. Then, the corresponding n quantities for all the frequency ranges can be determined at one time.
By using only one band-pass filter with a variable central frequency and bandwidth and only one averager, n-1 bandpass filters and n-1 averagers may be dispensed with in comparison with the preceding version of the first unit. However, this implies that the n quantities can no longer be determined simultaneously but have to be determined one after the other. Moreover, this leads to a more complicated control system.
The choice of the integration time of the averager should be such that it is not so long that the explosive character of speech is averaged out.
The correction means may comprise a similar band-pass filter, a similar averager and a signal combination unit, an input and an output of the band-pass filter being coupled to the input of the amplifier and to an input of the averager, respectively, which averager has an output coupled to a first input of the signal combination unit, a second input of the signal combination unit being coupled to an output of the averager of the first unit. By adjusting the integrating times of the averagers in the first unit and the correction means to the prescribed values, it can be ensured that compensation is obtained for the propagation delay of the acoustic signals in the space. This enables a suitable correction to be achieved by subtracting the signal on one input of the signal combination unit from the signal on the other input of this unit, provided that the two signals are applied to the two inputs in the correct phase relationship.
It is evident that, if the first unit comprises only one filter whose central frequency and bandwidth are adjustable to the central frequencies and bandwidths of the n frequency ranges, and if the correction means are constructed in the same way, then the filters should operate in identical frequency ranges.
It is also obvious that the correction means may be constructed differently, for example as the correction means described in DE-OS Nos. 25 22 381, 2 414 143, 27, 31 971, 24 56 468, and EP No. 26 929.
The device may be characterized further in that a second controllable amplifier is arranged between the second input terminal of the device and the input of the first unit. This step may be necessary because the dynamic range of the background noise may sometimes be very large. By providing additional attenuation before the first unit a simpler and less accurate analog-to-digital converter may be used if the first unit comprises an analog-to-digital converter for digitizing the signal before it is processed further.
The conversion means may be characterized in that they comprise
In the first mentioned case the sixth unit should convert the information for the n frequency ranges, which is stored in the fourth and the fifth unit, into a single control signal which controls the gain of the amplifier over the entire audio-frequency range. In this situation the input signal of the first unit is preferably first subjected to an A-weighting or another weighting operation. For more details about the term "A-weighting" reference is made to "Application of B & K Equipment to Acoustic noise measurements" a Bruel and Kjaer publication of June 1975, (reprint), see in particular Chapter 3.8, page 31 and Chapter 4.1, page 43.
This A-weighting can ensure that all frequency ranges contribute to the amplifier control signal to the correct extent.
In the second case (when m=n) the sixth unit converts the information for each of the n frequency ranges, which is stored in the fourth unit and the fifth unit, into a control signal for each of the n frequency ranges, by means of which control signals the gain of the amplifier in the n frequency ranges is corrected. If m is smaller than n, the information relating to m groups of the n frequency ranges from the fourth unit and the fifth unit is combined by the sixth unit to obtain m control signals.
In all the cases the output of the address counter is also coupled to an address input of the fourth unit and the fifth unit.
If the gain of the amplifier is variable and adjustable in n frequency ranges, the sixth unit may be adapted to derive n control signals for adjusting the gain of the amplifier in the n frequency ranges.
The contents of the memories of the fourth unit, being the n reference values each corresponding to a frequency range, may be obtained in an initialising process before the method is started. In this initialising process the gain of the amplifier is set manually (by means of the volume control). It is assumed that the operator adjusts the gain of the amplifier by means of the volume control so as to obtain a suitable level of the desired signal relative to the background noise. A quantity which is a measure of the gain of the amplifier corresponding to this suitable level is now stored in the fourth unit as a reference value. Controlling is then effected in such a way that, starting from this initial setting of the gain, the device adapts the gain in the case of a variation of the background-noise level in such a way that for an increased background level the difference in level (in dB) between the amplifier gain and the background level in the new situation is the same as in the original situation.
If the volume control is again adjusted by the operator (i.e.: if the operator sets the gain to another value relative to the prevailing noise level) the method may be re-started by applying the new reference values to the fourth unit.
A device for deriving a control signal from an electric signal for use in the method in accordance with the invention may be characterized in that it comprises:
If the device for deriving the control signal is for use in the method in which measurements in two or more frequency ranges are made, it may be characterized further in that the second unit comprises n memories each corresponding to a respective one of the n frequency ranges, each for storing a value which is a measure of the energy content of the electric signal in the associated frequency range, the first comparator is adapted to compare the value for the quantity corresponding to the i.sup.th frequency range with the value of the contents of the i.sup.th memory of the second unit, the third unit comprises n counters each corresponding to a respective one of the frequency ranges, the second comparator is adapted to compare the count of the i.sup.th counter with a first final value, the first comparator is further adapted to supply respective output signals if the value of the quantity corresponding to the i.sup.th frequency range lies within or outside the range of values around the value stored in the i.sup.th memory of the second unit respectively, and the second comparator is further adapted to supply a control signal if in the second comparator the count of the i.sup.th counter in the third unit is found to have reached the first final value, and the device further comprises an address counter having an output coupled to an address input of the second unit and the third unit.
This last-mentioned device may be characterized further in that a seventh unit is arranged between the output of the conversion means and the control input of the amplifier, which seventh unit is capable of storing the value of the control signal derived in the sixth unit and comprises a load input for receiving a control signal at least once during an operation if during this operation it is found that for at least one frequency range i the count of the counter i of the third unit has reached the first final value. In this way a non-recurrent adaptation of the gain can be obtained in one operation, for example at the end of each operation. This device may be characterized further in that the seventh unit is adapted to store n values corresponding to the n control signals for controlling the gain of the amplifier, and the seventh unit further comprises an address input coupled to the output of the address counter.
Parts of the devices in accordance with the invention may be integrated and incorporated in a microprocessor. Preferably, at least the second unit and the third unit, the first comparator and the second comparator, the conversion means, and the address counter form part of the microprocessor.
Embodiments of the invention will now be described in more detail, by way of example, with reference to the accompanying drawings. In the drawings: